Submerged CALM buoy

ABSTRACT

A Catenary Anchor Leg Mooring (CALM) buoy (1) comprises a body (2) with buoyancy, elements (3) for connecting the buoyant body to catenary anchor chains to anchor the body to the seabed, a turntable (5) rotatable elements with respect to the body with buoyancy. The turntable has devices (6, 6&#39;) for a floating unit such as a vessel, a swivel and elements for connecting at least one hose (8, 8&#39;) between at least one swivel and a submerged pipe structure extending over or into the seabed to any subsea well or the like, as well as elements for connecting at least one hose (9) between at least one swivel and at least one vessel (7), wherein the CALM buoy is provided with catenary anchor chains (4) which have a weight such that the CALM buoy as a whole is kept underwater at least during normal practice.

The invention relates to a Catenary Anchor Leg Mooring (CALM) buoycomprising a body with buoyancy, means for connecting the buoyant bodyto catenary anchor chains to anchor the body to the seabed, a turntablebeing rotatable with respect to the body with buoyancy, said turntablehaving mooring means for a floating unit such as a vessel, a swivel andmeans for connecting at least one hose between at least one swivel and asubmerged pipe structure extending over or into the seabed to any subseawell or the like, as well as means for connecting at least one hosebetween at least one swivel and at least one vessel.

A CALM buoy serves for mooring a floating unit, such as a vessel, andfor connecting said floating unit to a submerged pipe structureextending over or into the seabed to any subsea well or the like.

BACKGROUND OF THE INVENTION

CALM buoys are well known as being buoys with buoyancy floating at sealevel. Because of said floating at sea level the CALM buoys are affectedby severe loads caused by forces exerted upon the hawser lines by thevessel floating to and from the buoy forces exerted on the meansconnecting the buoy with the vessel, and forces exerted upon thecatenary chains anchoring the buoy to the seabed, by the action of wavesto the buoyant body of the buoy. As a rule they have own stability andtherefore volume.

To connect floating units, such as vessels, to a mooring buoy and to asubmerged pipe structure extending over or into the seabed to any subseawell or the like, several proposals are known from the prior art.

To solve the problems of these forces U.S. Pat. No. 3,823,432 and U.S.Pat. No. 3,908,212 both disclose a single point mooring buoy having abuoyant body which is small with respect to the floating unit, andaround which the floating unit can weathervane. The vessel and buoy areinterconnected by a rigid arm. The arm has a horizontally disposed hingeinterconnecting the floating unit and the buoyant body so that they canachieve an angular displacement at need with respect to each other in avertical common plane of symmetric. The size and thereby the mass of thebuoyant body is chosen so that it offers very little resistance to theaction of waves thereon. Vividly the mooring buoy will follow themotions of the floating unit as well as the motions of the sea surface.

However, the amplitude and frequency of the primarily cyclic motions ofthe floating unit and the sea surface differ substantially from oneanother. Such different motions may result in heavy loads exerted on thecatenary anchor lines as well as the hinge mentioned above.

The above mentioned known arrangements of a small buoy which by means ofa rigid arm is held by the vessel like a barrow, does already reducesaid loads under the majority of circumstances. However, it may occurthat the combination of forces on one hand of the vessel through arm andbuoy on part of the catenary lines, in particular if already driftedaway from its proper location, and on the other hand of the waves leadsto unexpected overloads. These loads appear to be that heavy that themaximum allowable tensions in the catenary anchor lines and in the hingewill be exceeded easily causing line failure and breaking of the pivotrespectively.

Alternatively, large semi-submersible mooring buoys are known, forinstance from U.S. Pat. No. 4,784,079. Such large structures areparticularly indifferent to the motions of the sea surface because oftheir size being relatively large in view of the amplitude of the waves.These structures are indifferent also to the motions of a vesselinterconnected thereto because of their size and mass which may be equalor even larger than those of said vessel. In U.S. Pat. No. 4,784,079 arather complex structure for interconnecting the buoyant body and thevessel is shown, primarily to cope with the motions of the floating unitrelative to the buoyant body. Said large structures involve high cost toproduce and to maintain these structures.

U.S. Pat. No. 4,617,000 discloses a much smaller buoy not beingconnected rigidly to a vessel, comprising a buoyant body with anchoringchains, a turntable on said body with a mooring for a vessel. Accordingto said publication the turntable has buoyancy as well and preferablytakes part in the water displacement of the buoy. The bearing betweenturntable and body is preferably above sealevel between a central sleeveof the body and the turntable, accessible from above and located in theline from mooring point on the turntable to chain stopper on the body.The buoy disclosed is a semi-submersible buoy.

The structure disclosed in U.S. Pat. No. 4,617,000 is particularlyaffected as well by the motions of the sea surface because of its sizebeing relatively less or equal in view of the amplitude of the waves,and the motions of the vessel interconnected thereto because of its sizeand mass being essentially less or equal in view of those of saidvessel.

Such affections may still lead to unexpected overload of the anchorlines or connecting arm. In practice said loads appear to be that heavythat the maximum allowable tensions in the catenary anchor lines as wellas in the connecting arm or hinge interconnecting said arm to the buoy,will be exceeded easily causing line failure and breaking of the arm orpivot respectively.

The object of the invention is to minimize the loads exerted on both thecatenary anchor lines and the mooring means caused by the motions of thesea surface and the floating unit relative to the buoyant body.

SUMMARY OF THE INVENTION

According to the invention the Catenary Anchor Leg Mooring (CALM) buoycomprising a body with buoyancy, means for connecting the buoyant bodyto catenary anchor chains to anchor the body to the seabed, a turntablebeing rotatable with respect to the body with buoyancy, said turntablehaving mooring means for a floating unit such as a vessel, a swivel andmeans for connecting at least one hose between at least one swivel and asubmerged pipe structure extending over or into the seabed to any subseawell or the like, as well as means for connecting at least one hosebetween at least one swivel and at least one vessel, is characterized inthat the catenary anchor chains which have a weight such that the CALMbuoy as a whole is kept underwater at least during normal practice.

According to a further aspect of the invention the catenary anchorchains have a weight such that the CALM buoy extends to no more than atleast one time the depth of a keel of a current vessel or the like,below current sea level while rising from seabed toward sea level.

Accordingly, contrary to normal practice and opinions the fully equipedbuoy is positioned below water level, preferably far below it. This hasa number of advantages. Primarily the buoy is no longer subjected to theforces of the waves.

For small floats this is well known but no expert ever thought about thequestion whether a mooring buoy of the described type could be placedbelow water level because of the need to service it.

Furthermore this allows positioning of the buoy in the line of forcesfrom the vessel towards the anchor or anchors at the particular sitewhich is more favourable under all conditions and water depths. This newposition allows for a different design, in particular with respect toits magnitude because wind and wave forces on the buoy no longer exist.

According to a further aspect of the invention the turntable isdetachably connected to the body with buoyancy. This may be a quickrelease coupling with guide means to allow servicing of the swivel andbearings.

According to a further aspect of the invention the mooring means areequipped with flotation means.

According to a further aspect of the invention the mooring means may belengthened such that when rising essentially in upright position fromthe buoyant body toward sea level, said means extends to just below orjust above sea level, so that a connection can easily be made.

According to a further aspect of the invention the mooring means is arigid arm which comprises at the buoyant end a pivotal connection to thebuoy and at the other end means for connecting a mooring hawser thereto.

To minimize the loads exerted on both the catenary anchor lines and thehinge caused by the motions of the sea surface the mooring buoy ispreferably configured as a submerged mooring buoy. The CALM buoyaccording to the present invention is particularly suited not to beaffected by the action of waves thereon. Hereby the loads exerted on thecatenary anchor lines connecting the CALM buoy to the seabed areminimized.

The vessel must be able to weathervane around the mooring buoy and tothis end the CALM buoy comprises two parts, being rotatable relative toeach other. For sake of simplified serviceability one of said parts canbe disconnected from the other part to easily elevate the disconnectedpart to above sea level.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further elucidated referring to an embodiment ofthe invention shown in the drawings, wherein:

FIG. 1 shows an embodiment of the invention in side view;

FIG. 2 shows on an enlarged scale the CALM buoy from FIG. 1 in sideview; and

FIG. 3 shows the CALM buoy from FIG. 2 in plan view.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 a CALM buoy 1 is shown floating below sea level 10, anchoredby catenary anchor lines 4 to a seabed, wherein a vessel 7 is connectedto mooring means 6, 6' of the buoy by a mooring hawser 14, 14' and to aswivel (not shown) by a hose 9 connected at its other end to a bowcoupler (not shown) at the vessel 7.

The CALM buoy comprises a buoyant body 2 forcing the buoy to rise towardsealevel. The buoy is kept underwater by the catenary anchor lines 4having a suitable weight as to withhold the buoy to float at sea level10. The buoyant body 2 of the CALM buoy 1 comprises means 3 such aschain tables for connecting said buoyant body to the catenary anchorchains 4.

Further, the CALM buoy 1 comprises a turntable 5 comprising mooringmeans 6, 6', for example, a rigid arm which comprises at the buoyant enda pivotal connection 11 to the turntable 5 and at the other end meansfor connecting a mooring hawser 14 thereto.

As mentioned, a hose 9 extending from the vessel 7 is connected to oneend of the swivel of the CALM buoy 1. At the other swivel end hoses 8are connected to extend between the swivel and a submerged pipestructure extending over or into the seabed to any subsea well or thelike.

In FIG. 2 the CALM buoy 1 from FIG. 1 is shown on an enlarged scale. TheCALM buoy 1 comprises a buoyant body 2, means 3 for connecting saidbuoyant body to the catenary anchor chains 4, a turntable 5 comprisingmooring means 6, 6' and means 12 for connecting a mooring hawser 14, 14'to said mooring means 6, 6', and a pivotal connection 11 to connect themooring means 6, 6' to the structure of the turntable 5.

Further the CALM buoy 1 comprises a connection means 13 for detachablyconnecting the turntable 5 to the buoyant body 2. The connection means13 are provided for sake of simplified serviceability.

As can be seen from FIG. 2, the CALM buoy 1 is connected to a mooringhawser 14 by means 12, and to a catenary anchor chain 4 by means 3.

In FIG. 3 the CALM buoy 1 from FIG. 2 is shown with the reference signsindicating the same elements as shown in FIG. 2.

Although the embodiment according to FIGS. 1 to 3 is most preferred atthe time being, it will be immediately clear that a lot of amendmentscan be introduced without leaving the scope of protection. These andother amendments which are obvious for the persons skilled in the art donot restrict the scope of protection of the application.

We claim:
 1. A catenary anchor leg mooring buoy comprising a buoyantbody, means for connecting the buoyant body to catenary anchor lines foranchoring the body to the seabed, the catenary anchor lines having aweight such that the buoy as a whole is kept underwater when the anchorlines are attached to the seabed, the buoy further comprising aturntable which is rotatable with respect to the buoyant body, saidturntable having a rigid mooring arm for connection to a floating vesselvia an anchor line or chain and for exerting a rotational moment on theturntable upon weathervaning of the vessel around the buoy, said mooringarm having a relatively short length such that when in an essentiallyupright position from the buoyant body toward sea level, the mooring armextends no further than to just below or just above the sea level, aswivel and means for connecting at least one hose between the swivel anda submerged pipe structure extending over or into the seabed to anysubsea well, and means for connecting at least one hose between theswivel and the floating vessel.
 2. Catenary anchor leg mooring buoyaccording to claim 1, wherein the weight of the catenary anchor lines issuch that the buoy extends to at least one time the depth of a keel ofthe floating vessel below the sea level.
 3. Catenary anchor leg mooringbuoy according to claim 1, wherein the turntable is detachably connectedto the buoyant body.
 4. Catenary anchor leg mooring buoy according toclaim 3, wherein the mooring arm is equipped with floatation means. 5.Catenary anchor leg mooring buoy according to claim 4, wherein themooring arm has a buoyant end and a second end, a pivotal connectionconnecting the buoyant end to the buoy, and means for connecting amooring hawser to the second end.